1. Field of the Invention
The present invention relates to power line communications (PLC), and more particularly, to placement of an inductive coupler around a cable having a shield or sheath of neutral conductors. The present invention is particularly advantageous in a case where the cable is an underground power line.
2. Description of the Related Art
In a power line communication system, a data signal may be coupled onto and off of a power line via an inductive coupler, such as described in U.S. Pat. No. 6,452,482. Inductive couplers may be placed around an uninsulated phase conductor or an insulated phase conductor. However, in underground power distribution systems, there is often no physical access to, or space around, a section of a phase conductor or a center conductor in the vicinity of a cable termination. Even when such access exists, work rules may require de-energizing the cable prior to attachment of the coupler. This process is inconvenient and requires personnel at both ends of a cable segment, and may sometimes affect service to power customers.
Many underground cables are built with a solid coaxial shield, grounded at each end, the main purpose of which is to provide shielding. Other underground cables are built with a plurality of wires wound spirally around an insulated core, where the plurality of wires serve as a neutral conductor. This shield or neutral conductor sheath is terminated just short of the end of a center conductor. The length of the unsheathed center conductor can be very short, often too short to allow installation of an inductive coupler.
In contrast, a shielded or sheathed cable segment located slightly away from the cable termination is generally available for coupler attachment while the cable is energized. However, efficiency of inductive coupling is reduced by the cable's shield or sheath. This is due to signal current induced in the shield or sheath, of magnitude similar to that in the center conductor, but of opposite phase. Since an output of the inductive coupler is proportional to a phasor sum of current passing through an aperture of the coupler, signal currents in the center conductor and shield or sheath conductors will tend to cancel, greatly reducing amplitude of a coupled signal.